Tunable optical filters are useful in situations requiring spectral analysis of an optical signal. They can also be used, however, as intra-cavity laser tuning elements or in tunable detectors, for example. One of the most common, modem applications for these devices is in wavelength division multiplexing (WDM) systems. WDM systems transmit multiple spectrally separated channels through a common optical fiber. This yields concomitant increases the data throughput that can be obtained from a single optical fiber. There are additional advantages associated with the ability to use a single fiber amplifier to amplify all of the channels on an optical link and its use as a platform for dynamic channel/wavelength routing.
Tunable filters that operate in these WDM systems must typically be high quality/high finesse devices. Currently proposed standards suggest channel spacings of 100 GigaHertz (GHz) to channel spacings as tight as 50 GHz in the ITU grid; some systems in development have spacing of 20 GHz and less. Tunable filter systems that operate in systems having such tight channel spacings must have correspondingly small passbands when operating as monitors, receivers, and routing devices.
Typically, the design of the tunable filters is based on a class of devices generally referred to as Fabry-Perot (FP) etalons. These devices have at least two highly reflective elements defining the Fabry-Perot cavity. The tunability functionality is provided by modulating the optical length of the cavity.
Since these tunable filters are typically incorporated into larger systems offering higher levels of functionality and because the Fabry-Perot cavity must be modulated over distances corresponding to the wavelength of light that it is filtering, typically around 1,000 to 2,000 nanometers (nm) in wavelength, microoptical electromechanical systems (MOEMS) technology is typically used to fabricate the tunable filters. The most common implementation pairs an electrostatically-deflectable reflective optical membrane with a fixed reflector. Thin film technology is typically used to obtain the reflectivity. High finesse systems can require dielectric mirrors having greater than seven layers.
A common metric for characterizing the quality of tunable filter systems is the side mode suppression ratio (SMSR). This is the ratio between the magnitude of the lowest order mode in the spectral plot of the filter""s characteristic and the magnitude of the next largest mode, which is typically the next higher order mode.
A general configuration for MOEMS tunable filter Fabry-Perot cavities is termed a curved-flat cavity. In such cavities, one of the reflectors is near planar and the other reflector is curved. If the curved reflector has a spherical profile, the cavity is sometimes referred to as a hemispherical cavity.
When hemispheric tunable filters are used, for example, the optical train surrounding the filter must be designed with the objective to control SMSR.
One solution to controlling SMSR used in some conventional MOEMS filter systems is to integrate the tunable filter into the larger optical system by locating it between two fiber pigtails; one fiber pigtail emits the optical signal to be filtered and the other fiber pigtail collects the filtered optical signal after its transmission through the tunable filter. The tunable filter is oriented to be orthogonal to the axis extending between the fiber endfaces.
As optical systems are developed that allow for higher levels of functionality in a single package, increased attention is directed to the co-design of the tunable filter element and surrounding optical system. This is especially true in systems utilizing free-space-interconnects between the tunable filter and other optical components in the system.
One parameter that affects the SMSR of a MOEMS filter system is mode size matching between the lowest order transverse mode of the tunable filter and the mode size of the light as it is launched into the tunable filter. The mode field diameter is a measure of the radial intensity distribution of radiation. Mode field diameter is measured by the ITU-T reference test method based on the far field scan technique. The intensity of the radiation reaching the photodiode is recorded as a function of angle; and from these data, the mode field diameter is calculated. According to one definition, weighted mean of the angular radial intensity distribution is used. If the mode size of the light that is launched into the filter is smaller or larger than the lowest order mode of the filter, higher order modes will be excited, thereby degrading the performance of the system.
The spectral output of a Fabry-Perot filter, in general, comprises multiple spectrally distributed peaks in the filter""s response to a broadband light source. These different peaks are attributable to the longitudinal mode orders of operation of the cavity and the cavity""s transverse spatial modes. The pattern of the peaks repeats itself spectrally with a periodicity that is related to the separation between the mirrors, termed the free spectral range. Within a given order of longitudinal mode operation, the frequency separation between transverse modes is related to the curvature of the mirrors. Specifically, for Hermite-Gaussian transverse modes the spectral separation between the lowest-order mode and any higher-order mode with mode number (n,m) are given by:                               Δ          ⁢                      xe2x80x83                    ⁢                      v            HOM                          =                  xe2x80x83                ⁢                              (                          n              +              m              +              1                        )                    ⁢                      arccos            ⁡                          [                                                sqrt                  ⁡                                      (                                          1                      -                                              L                                                  r                          1                                                                                      )                                                  ·                                  sqrt                  ⁡                                      (                                          1                      -                                              L                                                  r                          2                                                                                      )                                                              ]                                ⁢                      c            /                          (                              2                ⁢                π                ⁢                                  xe2x80x83                                ⁢                L                            )                                                              =                  xe2x80x83                ⁢                              (                          n              +              m              +              1                        )                    ⁢                                    arccos              ⁡                              [                                  sqrt                  ⁡                                      (                                                                  g                        1                                            +                                              g                        2                                                              )                                                  ]                                      ·                          c                              2                ⁢                π                ⁢                                  xe2x80x83                                ⁢                L                                              ⁢                      xe2x80x83                    ⁢          where          ⁢                      xe2x80x83                    ⁢                      g            1                                                            =                      xe2x80x83                    ⁢                                    1              -                                                L                  /                                      r                    1                                                  ⁢                                  xe2x80x83                                ⁢                and                ⁢                                  xe2x80x83                                ⁢                                  g                  2                                                      =                          1              -                              L                /                                  r                  2                                                                    ,                  where          ⁢                      xe2x80x83                    ⁢          r1                    
and r1 are the radii of curvature of the two mirrors and L is the mirror separation.
Typically, one of the mirrors will have a known radius of curvature, for example, in a curved-flat cavity. Such information can be determined using white-light interferometery or other surface profilometry. The other mirror""s radius can thus be computed.
This scheme is useful in the situation where the known mirror has a relatively small radius, and thus can be measured accurately. When the second mirror has a very long radius, it is difficult to measure its radius, especially if its effective aperture is small.
The present invention is directed to a technique for determining the mode size of a MOEMS tunable Fabry-Perot filter by reference to a calculated value for the curvatures of the reflectors that form the Fabry-Perot tunable filter cavity. Specifically, in the case of a concentric Fabry-Perot cavity or related cavity where one of the mirrors is relative flat, the curvature of the curved reflector is calculated from the spectral response of the tunable filter.
In general, according to one aspect, the invention features a process for configuring a tunable MOEMS filter train. The process comprises determining a spectral response of a MOEMS tunable filter. A spectral separation between different order longitudinal modes, or free spectral range, is then determined for the filter, as well as transverse mode spectral separation. This information is then related to a mode size of a desired mode of the tunable filter. With this information, lenses for the optical train are provisioned, and then installed so that light is launched into the optical filter at the desired mode size to thereby maximize the SMSR of the filter train.
In specific embodiments, the mode size of the injected optical signal is determined for the filter train. In the case of light being launched from a single mode optical fiber, the mode size is about 8-10 micrometers in diameter.
In one implementation, the spectral response of the tunable filter can be determined by tuning the tunable filter across a laser light source or other source that generates a spectrally narrow line. In another implementation, the filter spectral response is determined by injecting broadband xe2x80x9cwhitexe2x80x9d light into the filter and measuring the transmitted light spectrum.
According to other aspects of the preferred embodiment, the step of determining the spectral separation comprises determining a spectral separation between a lowest order mode and a next higher order mode within an order of operation of the tunable filter. Using this information, lenses in the optical train are selected to have beam forming characteristics that will yield the desired mode size at the tunable filter. These provisioned lenses are then installed in the filter train.
According to another implementation, the location of the lenses in the filter train can be adjusted to achieve the desired mode size at the tunable filter.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.